Emission control system with a catalyst

ABSTRACT

A reductant injection control strategy for controlling an amount of nitrogen oxide reducing agent injected upstream of a selective reduction catalyst uses an ammonia sensor located downstream of the catalyst. An open loop injection quantity is first determined based on operation conditions. Ammonia concentration detected downstream of the catalyst is controlled to a desired value, with the desired value based on catalyst temperature and the open loop injection quantity.

FIELD OF THE INVENTION

The invention relates to a system and method for controlling ammoniainjection upstream of a selective reduction catalyst for use with aninternal combustion engine.

BACKGROUND OF THE INVENTION

In order to meet some emission regulations, selective catalyticreduction systems using externally added reducing agents may be used. Insuch a system, regulated emissions, such as certain nitrogen oxides, orNOx, can be reduced in a oxygen-rich environment to nitrogen and waterover a catalyst when a reducing agent, such as ammonia, is added. Inaddition to controlling nitrogen oxide emissions, the amount of excessammonia, or ammonia slip, must be managed. Ammonia slip is experiencedwhen ammonia in excess of that used to reduce the nitrogen oxides passesthrough the catalyst unaffected and exits the catalyst (as ammoniaslip).

One method for regulating ammonia slip is to use an ammonia sensorlocated downstream of the catalyst. The detected ammonia concentrationis compared with a fixed upper threshold value. This comparisongenerates a correction signal that is used to control the metering ofammonia upstream of the catalyst. Allegedly, by regulating actualammonia slip to the upper threshold value, a certain nitrogen oxidereduction is obtained. Such a system is disclosed in U.S. Pat. No.5,369,956.

The inventors herein have recognized a disadvantage with the abovesystem. The above system regulates to a fixed concentration value forthe upper threshold ammonia slip. However, this system does not considerNOx conversion efficiency or percentage slip. While NH₃ slip expressedas concentration (ppm) and as a percent are related, there is animportant distinction in their use for reductant control strategy. Ingeneral, as maximum NOx conversion is approached with increasing ammoniaaddition (i.e., increasing NH₃/NOx mole ratio), ammonia starts to slip.After maximum NOx conversion is attained, ammonia slip increases morerapidly with increasing NH₃/NOx. For example, if ammonia slip isregulated to a constant concentration value, an ammonia setting highenough for sufficient NOx conversion at high NOx feed gas levels islikely excessive for low NOx feed gas levels, thereby wasting ammonia.Conversely, a setting at minimum detectable ammonia concentration islikely insufficient to provide high NOx conversion at high NOx feed gaslevels. Further, intermediate settings may still be insufficient toprovide high enough NOx conversion at high NOx feed gas levels. Thus,prior approaches can not achieve high NOx conversion with minimalammonia slip, particularly for vehicle engines where NOx concentrationlevels varies widely and quickly.

In other words, because a catalyst experiences widely varying levels ofengine NOx, controlling to an ammonia slip concentration results inwidely varying, and less than optimum, NOx conversion efficiency.

SUMMARY OF THE INVENTION

An object of the invention claimed herein is to provide a system andmethod for controlling ammonia injection upstream of a selectivereduction catalyst using an ammonia sensor located downstream of thecatalyst to keep ammonia slip low while achieving a high level of NOxconversion.

The above object is achieved and disadvantages of prior approachesovercome by a method for controlling a reductant injection into acatalyst coupled to an internal combustion engine, the method comprisingthe steps of: determining a temperature region in which the catalyst isoperating; generating a reductant injection quantity based on engineoperating conditions; generating a desired reductant slip based on acatalyst temperature and said reductant injection quantity; andadjusting said reductant injection quantity so that an actual reductantslip approaches said desired reductant slip.

By regulating reductant slip to a desired value that is a fraction ofinjected reductant, NOx conversion efficiency is kept high and moreconsistent throughout widely varying NOx concentration levels typicalfor diesel vehicles. Further, since the desired ammonia slip value isalso based on temperature, this additionally improves NOx conversion.

It is therefore possible to control ammonia slip with improved NOxreduction, particularly for vehicle engines where NOx concentrationlevels varies widely and quickly. In other words, when ammonia slip isregulated to a fraction of injected reductant, or ammonia, high NOxconversion is provided without excessive slip throughout the widelyvarying NOx feed gas concentrations.

An advantage of the present invention is improved NOx conversion whilekeeping ammonia slip low.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

FIG. 1 is a block diagram of an embodiment wherein the invention is usedto advantage; and

FIGS. 2-3 are high level flow charts of various operations performed bya portion of the embodiment shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Internal combustion engine 10, comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1, is controlled by electronic enginecontroller 12. Engine 10 includes combustion chamber 30 and cylinderwalls 32 with piston 36 positioned therein and connected to crankshaft40. Combustion chamber 30 is known communicating with intake manifold 44and exhaust manifold 48 via respective intake valve 52 and exhaust valve54. Intake manifold 44 is also shown having fuel injector 80 coupledthereto for delivering liquid fuel in proportion to the pulse width ofsignal FPW from controller 12. Both fuel quantity, controlled by signalFPW and injection timing are adjustable. Fuel is delivered to fuelinjector 80 by a conventional fuel system (not shown) including a fueltank, fuel pump, and fuel rail (not shown). Alternatively, the enginemay be configured such that the fuel is injected directly into thecylinder of the engine, which is known to those skilled in the art as adirect injection engine.

Reducing agent, for example, ammonia, is stored in storage vessel 130coupled to exhaust manifold 48 upstream of catalyst 97. Control valve134 controls the quantity of reducing agent delivered to the exhaustgases entering catalyst 97. Pump 132 pressurizes the reducing agentsupplied to control valve 134. Both Pump 132 and control valve 134 arecontrolled by controller 12. Ammonia sensor 140 is shown coupled toexhaust manifold 48 downstream of catalyst 97. Temperature sensor 142coupled to catalyst 97 provides an indication of the temperature (T) ofcatalyst 97. Alternatively, catalyst temperature (T) could be estimatedusing methods known to those skilled in the art and suggested by thisdisclosure. Ammonia sensor 140 provides an indication of ammoniaconcentration [NH₃] to controller 12 for determining a control signalsent to control valve 134 as described later herein with particularreference to FIGS. 2-3.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:engine coolant temperature (ECT) from temperature sensor 112 coupled tocooling sleeve 114; a measurement of manifold pressure (MAP) frompressure sensor 116 coupled to intake manifold 44; a measurement (AT) ofmanifold temperature from temperature sensor 117; an engine speed signal(RPM) from engine speed sensor 118 coupled to crankshaft 40.

Referring now to FIG. 2, a routine for determining a control signal forcontrol valve 134 for controlling reductant addition is described.Reductant is ammonia in a preferred embodiment, but can be any nitrogen(N) containing substance, such as, for example, urea. During step 200, adetermination is made as to whether temperature (T) of catalyst 97 isbelow first threshold temperature T1. Calculation of first thresholdtemperature T1 is described later herein with particular reference toFIG. 3. When the answer to step 200 is YES, the desired mole ratio(Rdes) is set to zero in step 201 and the total quantity of reductant(Qtot) to be injected by control valve 134 is set to zero in step 203.Thus no reductant is added to the exhaust gases entering catalyst 97 togive a mole ratio (R) equal to first desired mole Ratio (R1) of zero.

Mole ratio (R) is the ratio of the number of moles of nitrogen (N) inthe reductant to the number of moles of nitrogen oxide in engine outexhaust gas. The moles of nitrogen oxide in engine out exhaust gas iscalculated based on experimentally determined relationships betweennitrogen oxide quantity and engine operating conditions known to thoseskilled in the art to be indicative of estimated engine out nitrogenoxide quantity (Nox^(est)) such as, for example, engine speed, manifoldpressure (MAP), intake air temperature (AT), injection timing, injectionquantity (FPW), and engine coolant temperature (ECT).

When the answer to step 200 is NO, a determination is made in step 204as to whether temperature (T) is below second threshold temperature T2.Calculation of second threshold temperature T2 is described later hereinwith particular reference to FIG. 3.

When the answer to step 204 is NO, a determination is made in step 208as to whether temperature (T) is below third threshold temperature T3.Calculation of third threshold temperature T3 is described later hereinwith particular reference to FIG. 3. When the answer to step 208 is YES,the desired mole ratio (Rdes) is set to third desired mole ratio (R3) instep 210. Then, in step 212, a determination is made as to whether themeasured ammonia concentration from sensor 140 is less than limit amountFR1. First limit amount FR1 is based on a fraction of reductant quantitypreviously injected. Further, first limit amount FR1 is determined forthe specific temperature range. Alternatively, first limit amount FR1can be a ratio of ammonia slip concentration to engine out (orcatalyst-in) NOx quantity. Thus, according to the present invention, theammonia slip is kept within a limit where the limit is a fraction of theamount of injected reductant.

Continuing with FIG. 2, if the answer to step 212 is YES, then in step214, adjusted reductant quantity (DQ) is set to a positive calibrationamount (r). If the answer to step 212 is NO, then in step 218 adjustedreductant quantity (DQ) is set to a negative calibration amount (−r).Then, from either step 214 or 218, the base reductant quantity (Qbase)is determined from the product of the desired mole ratio (Rdes) and theestimated engine nitrogen oxide production (Nox^(est)) in step 220.

When the answer to step 208 is NO, a determination is made in step 226as to whether temperature (T) is below fourth threshold temperature T4.Calculation of fourth threshold temperature T4 is described later hereinwith particular reference to FIG. 3. When the answer in step 226 is YES,the desired mole ratio (Rdes) is set to fourth desired mole ratio (R4)in step 228. Then, a determination is made in step 230 as to whether themeasured ammonia concentration from sensor 140 is greater than secondlimit amount FR2. Limit amount FR2 is calculated as a second fraction ofreductant quantity previously injected. In a preferred embodiment,second limit amount FR2 is less than first limit amount FR1. In analternative embodiment, limit amounts FR1 and FR2 can be set to constantlevels or adjusted to give a specified parts per million (ppm) ofammonia slip. Further, if urea were used in place of ammonia,appropriate adjustment of the fractions is needed to account for thedifferent molecular structure. Alternatively, second limit amount FR2can also be a ratio of ammonia slip concentration to engine out (orcatalyst-in) NOx concentration. According to the present invention,different limit amounts (FR1 and FR2) are used in different temperatureranges to maximize NOx conversion and minimize ammonia slip.

Continuing with FIG. 2, if the answer to step 230 is YES, then in step218 adjusted reductant quantity (DQ) is set to a negative calibrationamount (−r). Otherwise, adjusted reductant quantity (DQ) is set to apositive calibration amount (−r) in step 214.

When the answer to step 204 is YES, the desired mole ratio (Rdes) is setto second desired mole ratio (R2) in step 236. Then in step 232 adjustedreductant quantity (DQ) is set zero. Then, base reductant quantity(Qbase) is determined from the product of the desired mole ratio (Rdes)and the estimated engine nitrogen oxide production (Nox^(est)) in step220. Then, in step 222, total desired reductant quantity (Qtot) isdetermined from the sum of the base reductant quantity (Qbase) and theadjusted reductant quantity (DQ). The total desired reductant quantity(Qtot) is converted to a control signal sent to control valve 134 fordelivering the reductant in proportional thereto.

In this way, open loop reductant control is used to calculated the basereductant quantity (Qbase) from the product of the desired mole ratio(Rdes) and the estimated engine nitrogen oxide quantity (Nox^(est)).Also, desired mole ratio is adjusted based on catalyst temperature (T)to account for changes in catalyst efficiency.

Adjustment is made to this open loop value in two temperature rangeswhen the measured ammonia concentration from sensor 140 deviates from adesired value based on a fraction of reductant injection. Limit valuesFR1 and FR2 represent the allowable limits of ammonia slip. Thus, thereductant is controlled for maximum nitrogen oxide conversion withminimum slip. In an alternative embodiment (not shown), differentcalibration amounts can be used in different temperature ranges.Further, positive and negative calibration amounts can be different (notshown).

Referring now to FIG. 3, a routine for calculating temperaturethresholds is now described. First based temperatures (T1B, . . . , T4B)are determined based on predetermined calibration values in step 310.Then in step 312, the space velocity (SV) of the exhaust gas flowentering catalyst 97 is calculated based on the mass flow rate (m),density (r), and catalyst Volume (V). Then, in step 314, adjustmentvalues, (KA1, . . . , KA4), are determined based on space velocity (SV)of the flow entering catalyst 97 and calibration functions (f1 . . .f4). In a preferred embodiment, functions f1 . . . f4 act to reducetemperatures as space velocity decreases and increase temperatures asspace velocity increases.

Although one example of an embodiment which practices the invention hasbeen described herein, there are numerous other examples which couldalso be described. For example, the invention may be used to advantagewith both lean burning diesel and gasoline engines in which nitrogenoxide emissions are produced. Further, the present invention can be usedin diagnostic applications where the The invention is therefore to bedefined only in accordance with the following claims.

We claim:
 1. A method for controlling a reductant injection into acatalyst coupled to an internal combustion engine, the methodcomprising: determining a temperature region in which the catalyst isoperating; generating a reductant injection quantity based on engineoperating conditions; generating a desired reductant slip based on acatalyst temperature and said reductant injection quantity; andadjusting said reductant injection quantity so that an actual reductantslip approaches said desired reductant slip, wherein adjusting saidreductant injection quantity further comprises determining said actualreductant slip based on a sensor located downstream of the catalyst, anddecreasing said reductant infection quantity when said actual reductantslip is greater than said desired reductant slip.
 2. The method recitedin claim 3 wherein the reductant is any ammonia generating material. 3.The method recited in claim 3 wherein said step of generating saiddesired value of reductant slip further comprises generating saiddesired value of reductant slip based on a fraction of said reductantinjection quantity.
 4. The method recited in claim 3 further comprisingthe step of generating said desired value of reductant slip based on Noxgenerated by the engine.
 5. The method recited in claim 3 wherein saidfraction is based on said catalyst temperature.
 6. The method recited inclaim 3 wherein said step of generating said reductant injectionquantity further comprises the step of generating said reductantinjection quantity based on a catalyst temperature and a fraction of anengine out nitrogen oxide production.
 7. The method recited in claim 3further comprising the step of discontinuing said adjustment step basedon whether said catalyst temperature is within temperature limits. 8.The method recited in claim 7 wherein said step temperature limits arebased on exhaust gas space velocity.